Control system for eddy current coupling drive for armoured face conveyors

ABSTRACT

A control system for eddy current coupling drive for armoured face conveyors wherein the systems controls a number of eddy current drives via a pair of power supply lines and a single pilot line. A main contactor controlled by a relay is held closed by contact of a pilot relay which is energized only if relay contacts in individual control units for each drive remain closed. These last mentioned contacts open if certain operating conditions are not met.

BACKGROUND AND OBJECTS OF THE INVENTION

The present invention relates to a control system for an eddy currentcoupling drive for armoured face conveyors.

It is known to use eddy current coupling drives for conveyors and suchcouplings provide a controlled build up of torque and/or speed, andenable the load to be shared reasonably equally between drives when morethan one drive is used on the conveyor. When applying the eddy currentcoupling drive to armoured face conveyors, account must be taken of thefact that the weight of excavated material on the conveyor can increasethe frictional resistance and inertia such that considerable excesstorque may be required to start the conveyor. It may not be possible tocontrol, consistently, the required torque with the available eddycurrent coupling drive control system and in this situation the motorsmay stall. The supply voltages to the motors of several coupling drivesmay not be identical, resulting in varying maximum available torques,and thus making the application of maximum total torque more difficult.In addition, in longwall mining the drives may be at each end of thearmoured conveyor, and these can be separated by several hundred metres.Normally, each eddy current coupling is supplied from a separate controlunit which incorporates some form of control of a thyristor rectifierwhich supplies d.c. current to the excitation coils of the coupling. Ina multi-motor drive as is used in armoured face conveyors, these controlunits are interlinked electrically, and may be mounted in a commonenclosure with supply cables being connected to each coupling. If it isalso desired to monitor speed signals from the coupling drive, thisnormally requires another cable between the control units and eachcoupling drive.

Similarly, measurement of the motor terminal voltage as a controlparameter requires further cable between the coupling drives and thecontrol unit. This accumulation of cables between the control units andthe drives is impractical in the use of an armoured face conveyor.

It is an object of the present invention to obviate or mitigate thelimitations of the existing conveyor control system.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a control systemfor multiple eddy current coupling drives comprising:

an electrical power supply means including first switching meansactuable to close, in response to a start signal, a contact in thecircuit of a second switching means, and a timing means arranged tocomplete the circuit of the second switching means for a predeterminedperiod thus enabling power to be supplied to the several couplingdrives, and thereby energising a retaining relay in each coupling drive,said retaining relays having contacts being connected in series in aretaining circuit arranged to retain the said second switching meansafter said predetermined period has elapsed,

electronic control means integral with, or in close proximity to, eachcoupling drive and arranged to automatically initiate an increase ofexcitation applied to each coupling drive from an initial zero value toa preset maximum value over a preset time when power is applied to eachcoupling drive, said control means including means for inhibiting theincrease in excitation in response to input speed signals, speed sensingmeans for monitoring the input and output speeds of the coupling drive,said input speed being compared with a predetermined level, saidincrease in excitation being inhibited when said input speed falls tothe predetermined level, and

means for de-energising at least one retaining relay in response totiming means initiated by an inhibiting signal such that said at leastone relay is de-energised after a preset time unless an output speedsignal indicates that the conveyor is beginning to accelerate, thusinterrupting said retaining circuit and interrupting power supply to allcouplings.

Preferably said first and said second switching means include first andsecond relays respectively.

Alternatively, said first and said second switching elements may be anysuitable devices such as solid state switches.

Preferably also, the control means includes electrical powertransforming means for converting the main electrical power supply tothe operating supply level of the eddy current coupling devices.

Preferably also, the power transforming means includes fault detectingmeans, the fault detecting means being connected between the secondarywinding of the transformer and earth.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram of the control system; and

FIG. 2 is a schematic block diagram of the electronic control unit ofone eddy current drive in the system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1, a 550/1100 V main power supply 1a is connectedvia an isolator switch 1 to a step-down transformer 2. The transformer 2has two secondary windings 2a and 2b. The ratio of the primary windingto the secondary winding 2a determines the 415 V excitation supplynecessary to power the eddy current coupling drives (not shown). Acontrol contractor 3 is connected in the 415 V supply lines 12, and itsoperating coil 5 is connected across these lines in series with relaycontacts 14. An earth leakage relay 4 protects the eddy current couplingdrive against earth faults and is connected between the centre tap ofthe secondary winding 2a and earth. The energising of relay 4 by anearth fault causes normally closed contact 15 to open in a circuit 10 ofa pilot relay 11 to open contactor 3 and disconnect supply to theseveral coupling drives. The control of the contactor 3 is by a controlcircuit 6a which can be closed by a manual switch or a sequence contactin the motor starter. When this control circuit is closed, control relay6 is energised closing its contacts 7 and 8, contact 7 closing in thecircuit 10 of pilot relay 11 and contact 8 closing to energise a timingrelay 9 causing timing relay contacts 9a to close transiently andenergise pilot relay 11. Relays 6 and 11 are supplied from an auxiliarytransformer 16 which is in turn supplied from secondary 2b on thetransformer 2, while the timing relay 9 is supplied from a suitabletapping on the same secondary 2b.

When the pilot relay 11 is energised, contacts 14 close causing thecontactor operating coil 5 to be energised thus closing the contactor 3and applying power to the several coupling drives via supply lines 12.Once power is supplied to the several coupling drives this causes aretaining relay in each coupling to be energised (as will be described),closing relay contacts 13 in each coupling thus completing the retainingcircuit of pilot relay 11 by way of a pilot conductor 20 incorporated inthe same cable as the power supply, which is looped from the powersupply unit through the several coupling drives on the conveyor. Whenthe timing relay 9 times out, timing relay contacts 9a open but power tothe several coupling drives is maintained by the retaining circuit ofthe pilot relay 11.

Referring to FIG. 2, the field coil 21 of the eddy current coupling iscontrolled by a field controller 22. As is known per se, the fieldcontroller 22 supplies an excitation voltage to the field coil 21 whichincreases linearly over a given time from energisation of the supplylines 12 until full value is reached. Field controller 22 has an inhibitinput 23 which is controlled by a comparator 24 which operates tocompare an input speed signal on line 25 with an input speed referencesignal derived from potentiometer 26. The input speed signal on line 25represents the input speed of the coupling, that is the speed of themotor driving the coupling, and may be derived by any suitable meanssuch as a tachogenerator. If the input speed drops to be equal to orless than the reference, comparator 24 applies an inhibit signal toinput 23 to inhibit any further increase in excitation while thiscondition lasts, thus preventing stalling of the drive motor.

The retaining relay contact 13 is controlled by circuit 27 whichresponds to energisation of the supply lines 12 to energise associatedrelay coil 13a for a given time period. Thereafter, retaining relaycontrol circuit 27 maintains energisation of relay coil 13a independence on a comparator 28 which compares an output speed signal online 29 with an output speed signal derived from potentiometer 30, suchthat the relay coil 13a is de-energised if the output speed is less thanthe reference. Thus, if the output does not reach a preset speed withina preset time, contacts 13 open and the whole system is switched off.

Without departing from the scope of the invention it will be understoodthat the control system can be adapted for use with any other suitablepower supply, by altering the transformer ratio.

An important advantage of the present invention is that the availabletorque from each drive motor is utilised to the maximum independent ofthe local supply voltage to the motor. In addition, a further advantageis that the entire system requires only a signal a.c. power supply whichsupplies all the coupling drives via one cable along the length of theworking face; the cable carrying the power and control signals, and anearth return.

In addition, the features of the present invention would be incorporatedwith regard to the safety requirements specified and being of flameproofconstruction.

I claim:
 1. A control system for multiple eddy current coupling drivescomprising:an electrical power supply means including first switchingmeans actuable to close, in response to a start signal, a contact in thecircuit of a second switching means, and a timing means arranged tocomplete the circuit of the second switching means for a predeterminedperiod thus enabling power to be supplied to the several couplingdrives, and thereby energising a retaining relay in each coupling drive,said retaining relays having contacts being connected in series in aretaining circuit arranged to retain the said second switching meansafter said predetermined period has elapsed, electronic control meansintegral with, or in close proximity to, each coupling drive andarranged to automatically initiate an increase of excitation applied toeach coupling drive from an initial zero value to a preset maximum valueover a preset time when power is applied to each coupling drive, saidcontrol means including means for inhibiting the increase in excitationin response to input speed signals, speed sensing means for monitoringthe input and output speeds of the coupling drive, said input speedbeing compared with a predetermined level, said increase in excitationbeing inhibited when said input speed falls to the predetermined level,and means for de-energising at least one retaining relay in response totiming means initiated by an inhibiting signal such that said at leastone relay is de-energised after a preset time unless an output speedsignal indicates that a conveyor is beginning to accelerate, thusinterrupting said retaining circuit and interrupting power supply to allcouplings.
 2. The control system of claim 1, in which said first andsecond switching means comprise first and second relays, respectively.3. The control system of claim 1 or claim 2, in which the control meansincludes electrical power transforming means for converting the mainelectrical power supply to the operating supply level of the eddycurrent coupling devices.
 4. The control system of claim 3, in which thepower transforming means includes fault detecting means, the faultdetecting means being connected between the secondary winding of thetransformer and earth.